Normally-black-mode LCD devices are known which include a LC cell including a LC layer having a homogeneous initial orientation and a pair of transparent (glass) substrates, and a pair of polarizing films having optical axes extending perpendicular to each other. This type of LCD devices include an IPS (in-plane-switching)-mode LCD device and a FFS (fringe-field switching)-mode LCD device. These LCD devices operate for image display by applying an electric field to the LC layer in a direction parallel to the glass substrates to control the orientation of the LC layer. The configuration of the IPS-mode or FFS-mode LCD device wherein the orientation of the LC layer is parallel to the glass substrates allows the LCD device to achieve a higher viewing angle characteristic compared to a TN (twisted-nematic)-mode LCD device.
It is known that a leakage light and/or chromaticity shift (coloring) is observed in the IPS-mode or FFS-mode LCD device, as viewed in the direction of an azimuth angle of 45 degrees, for example, with respect to the polarization direction of the pair of polarizing films during display of a dark state (black). There is a known technique for solving this problem by using an optical compensation film, which suppresses the leakage of light and chromaticity shift as viewed in a slanted viewing direction during display of a dark state (for example, refer to Patent Publication JP-2005-196149A (Patent Publication-1)).
FIG. 8 shows the structure of the LCD device described in Patent Publication-1. In FIG. 8 and other accompanying drawings in this application, a solid line shown on a layer (film) represents an optical axis or light transmission axis of the corresponding layer, and a dotted line on a film represents a light absorption axis of the corresponding polarizing film. A blank arrow indicates the direction of backlight incident onto the LCD device.
The LCD device includes a LC cell 210 including a homogeneously-oriented LC layer (not shown) and a pair of glass substrates (not shown) sandwiching therebetween the LC layer. A light-emitting-side polarizing film 204 includes a polarizer 201 configured by a PVA (polyvinyl alcohol) layer, and a pair of protective layers 202, 203 sandwiching therebetween the polarizer 201. A light-incident-side polarizing film 208 includes a polarizer 206 and a pair of protective layers 205, 207 sandwiching therebetween the polarizer 206.
The optical axis (light absorption axis or light transmission axis) of the light-emitting-side polarizing film 204 and optical axis of the light-incident-side polarizing film 208 extend perpendicular to each other. The light absorption axis of the light-incident-side polarizing film 208 and the initial orientation of the LC cell 210 are substantially parallel to each other. The protective layers 202, 203, 207 have an optical axis in the thickness direction thereof, and have a retardation of about 50 nm in the thickness direction thereof. Similarly, the protective layer 205 has an optical axis in the thickness direction thereof, and has a retardation of 0 to 25 nm in the thickness direction thereof.
An optical compensation layer 214 is disposed between the light-emitting-side polarizing film 204 and the LC cell 210. The optical compensation layer 214 has a biaxial anisotropy wherein the refractive index (ns) of the in-plane slow axis of the optical compensation layer 214, the refractive index (nf) of the in-plane fast axis thereof, and the refractive index (nz) in the thickness direction thereof satisfy therebetween the relationship of (ns−nz)/(ns−nf)≦0.5 and the in-plane retardation Re of the optical compensation layer 214 is in the range of 80 nm≦Re≦230 nm. This optical compensation layer 214 is disposed so that the in-plane slow axis is parallel to the initial orientation of the LC layer. It is recited in Patent Publication-1 that the retardation caused in the slanted viewing direction by the protective layer 205 is isotropic and set at a lower value, preferably at 0 nm, whereby the light passed by polarizer 206 is allowed to have a linear polarization during incidence thereof onto the LC layer. The configuration wherein the orientation of the LC layer is parallel to the light incidence surface prevents change of the polarization of incident light, to thereby suppress the chromaticity shift thereof.
In the technique of Patent Publication-1, the range of chromaticity shift is reduced in the slanted viewing direction during display of a dark state. However, there remains a wavelength dependency of the biaxial anisotropy of the optical compensation layer 214 because the optical compensation is achieved only by the optical compensation layer 214 having a positive dispersion characteristic in the birefringence. It should be noted in this configuration that the chromaticity shift arises due to the change of spectrum of the backlight or transmission spectrum of color filters, and thus the retardation of the optical compensation layer having the biaxial anisotropy is adjusted so as to obtain an optimum chromaticity. However, this adjustment of retardation cannot completely remove the chromaticity shift if the blue range and red range in the spectrum of the backlight have therebetween considerably different peak values. In such a case, the wavelength dependency of the optical compensation layer having the biaxial anisotropy must be reduced or completely removed in an ideal case. The removal of the wavelength dependency requires a material having a reverse dispersion characteristic to achieve a smaller retardation in a shorter-wavelength range of the light, and the optical compensation layer having the above configuration is difficult to manufacture from such a material.